Devices that monitor and visualize information relating to the dynamics of a system detect forces that cause the motion of the system relative to three axes of a reference frame. These forces can be used to recreate the motion of the system, and consequently to render an image of that motion, as a projection in a virtual three-dimensional display space, as a projection or intersection with a two-dimensional plane, or as an oscillation along a single axis. The reconstruction is possible in any environment, whether the system moves in three-dimensional space, on a surface, or along a single line.
How accurately images can be visualized or reconstructed depends on how completely the detected forces describe the motion of the system in its environment. For example, forces applied along more than one axis to a pen point may represent that the pen point moved along a writing surface, or may represent that the pen is being pressed at an angle on a stationary point. Additional information is necessary to distinguish between these situations so that an accurate image can be obtained. Similarly, the motion of the head of a golf club can only be accurately visualized if all forces executed upon it are known, including the torsion and the strain on the shaft of the club caused by inertia of the club head and air resistance. The ability to image dynamic information regarding a system, therefore, is limited by the available information regarding variation of motion and orientation of the components of the system.
In general, devices for imaging information of systems in motion use external reference points to measure position and orientation, at various moments in time. For instance, instruments that measure the position of a satellite measure orientation with respect to the earth's horizon and with respect to a distant star. As another example, pen computers use a tablet to measure the change of position and orientation of a pen point on a writing surface. However, a suitable sensor system can be incorporated into a device in order to measure the forces acting on an object that alter its position or orientation.
Force, like many physical phenomena, cannot be measured without disturbing the phenomenon being measured. Most force transducers have an elastic sensing element, whose deformation is a measure of the acting force. In many force measurement systems, such as strain gauges, inductive, and capacitive systems, this deformation itself must be measured. The sensing element must be compliant enough to provide sufficiently large deformation and hence useful sensitivity. However, large deformations are undesirable because they limit the frequency response of the measuring system and also introduce geometric changes into the force measuring path which inevitably leads to measurement errors.
Piezoelectric materials, which can convert forces into electricity, are useful for detecting forces. In piezoelectric force transducers, the sensing element is the same as the transduction element which produces the electrical output signal from an acting force. Therefore, it is not necessary to measure the deformation, which is typically much smaller than with other measuring systems. The resultant rigidity of piezoelectric force transducers greatly reduces the distortion caused by the measurement and provides an inherently high natural frequency and associated rise time. This permits the measurement of extremely fast events that otherwise might be difficult to discern accurately.
Piezoelectric accelerometers require the addition of an inertial mass to a piezoelectric force transducer. As the mass is accelerated, it exerts a force on the piezoelectric material. Because of the constant inertial mass, the force acting on the measuring element is proportional to the acceleration in accordance with Newton's first law. Thus, the electrical charge generated by the piezoelectric material is proportional to the acceleration.
Piezoelectric components are also capable of converting electricity into force, and thus can be used as actuators. In its simplest form, a piezoelectric actuator abuts against a non-displaceable support and pushes against a displaceable element. When an electric voltage is applied across the piezoelectric element, it expands, displacing the displaceable element. The variations in length tend to be rather small, even when individual piezoelectric elements are arranged in stacks with an overall height of approximately 20 mm. Such arrangements are used as precision drives, for example, in adjustment operations.
However, existing piezoelectric devices are unable to measure forces or accelerations in three axes. What is needed is a device capable of measuring and/or transmitting forces along multiple axes in a sensitive, controlled manner. What is also needed is a device capable of sensitive acceleration measurements in multiple axes. A device that combines sensor and actuator capabilities for sensitive, controlled feedback mechanisms is also needed.